1. Field of the Invention
This invention relates to a paper feed tractor for supplying paper to a printer for use with a computer, a word processor, a plotter and so forth or to a printer of various other types.
2. Description of the Prior Art
Paper having a pair of feed holes or perforations formed therein along the opposite longitudinal edges thereof is used very frequently on a printer for use with a computer, a word processor, a plotter or the like as paper on which characters or figures outputted from the printer are to be recorded. Such feed holes are formed at fixed intervals on straight lines along the opposite longitudinal edges of the paper at locations spaced by a predetermined distance from the opposite longitudinal edges of the paper and are adapted to be engaged with feed pins which are provided projectingly on a belt of a paper feed tractor at locations corresponding to the opposite longitudinal edges of the paper.
FIGS. 24 to 26 show a conventional paper feed tractor. Referring to FIGS. 24 to 26, the paper feed tractor generally denoted at 1000 includes a main frame 1100 and a sub frame 1200 both made of a plastic material or the like and disposed in a spaced relationship by a predetermined distance from each other. The two frames 1100 and 1200 are integrated with each other by means of a fastening screw 1250 or the like.
An endless belt 1300 made of a rubber or plastic material is supported between the main frame 1100 and the sub frame 1200 for driving movement by a sprocket wheel (not shown) and has a large number of feed pins 1350 provided projectingly on an outer periphery thereof at intervals equal to the intervals of feed holes of paper. A pair of circular through holes H.sub.1 and H.sub.2 are formed in each of the main frame 1100 and the sub frame 1200, and a hub portion 1800 formed on the sprocket wheel is supported for rotation in the through holes H.sub.1 of the main frame 1100 and the sub frame 1200 through which a drive shaft extends. The hub portion 1800 of the sprocket wheel functions as a rotary shaft of the sprocket wheel and has a square through hole formed at a location around a center axis of rotation thereof. The sprocket wheel has teeth formed on an outer periphery thereof for engagement with teeth formed on an inner periphery of the belt 1300 to drive the belt 1300.
The main frame 1100 has a pair of supporting portions 1150 formed thereon for receiving and supporting thereon a shaft 1450 of a cover 1400 which has a paper holder 1460 thereon. A spring 1600 extends between the main frame 1100 and the cover 1400 and normally urges the cover 1400 in its closing direction.
The holes H.sub.2 of the main frame 1100 and the sub frame 1200 are adapted to receive a support shaft therein and have a fixing device 1700 provided thereon. The fixing device 1700 includes a collet which clamps the support shaft as a ring 1750 is turned by a certain angle in a direction in order to secure the paper feed tractor 1000 at an arbitrary position on the support shaft.
FIG. 27 shows such paper feed tractors 1000 as described above installed in a printer. In particular, a pair of such paper feed tractors 1000 are mounted at left and right positions on a pair of drive shaft S.sub.1 and a support shaft S.sub.2 which are disposed in such a manner as to provide an interaxial distance L therebetween in the printer. The drive shaft S.sub.1 is supported for rotation on a frame of the printer by means of a pair of left and right bearings B.sub.1 while the support shaft S.sub.2 are secured to a pair of left and right shaft receivers B.sub.2 provided on the printer frame.
The drive shaft S.sub.1 is provided to transmit a turning force and has a square cross section to allow the drive shaft S.sub.1 to engage with the square through holes of the sprocket wheels of the paper feed tractors 1000. Meanwhile, the support shaft S.sub.2 is adapted to slidably engage with the holes H.sub.2 formed in the frames 1100 and 1200 of the paper feed tractors 1000 and has a circular cross section.
The left and right paper feed tractors 1000 are each positioned in the drive and support shafts S.sub.1 and S.sub.2 as shown in FIGS. 27 and 28 while the portions thereof are adjustable in the direction indicated by an arrow mark X parallel to the direction of the axes of the drive shaft S.sub.1 and the support shaft S.sub.2 in accordance with the width and the position of paper to be fed by the paper feed tractors 1000. The span between the two bearings B.sub.1 and the span between the two shaft receivers B.sub.2 on which the two shafts S.sub.1 and S.sub.2 are supported, respectively, is great where the printer is of a large size. As the size of a line printer or the like increases recently, the amount of deflection of a drive shaft such as the drive shaft S.sub.1 is apt to increase.
FIG. 28 shows the drive shaft S.sub.1 the center axis CS.sub.1 or rotation of which is deflected by a maximum amount d.sub.0 from a straight line C.sub.0 interconnecting the two bearings B.sub.1. In the condition, if the drive shaft S.sub.1 is driven to rotate in the direction indicated by an arrow mark R around the center axis of rotation thereof, it will rotate with a deflection amount d at a location of each of the paper feed tractors 1000. Consequently, the interaxial distance L between the center axis CS.sub.1 of the drive shaft S.sub.1 and the center axis CS.sub.2 of the support shaft S.sub.2 varies during rotation of the drive shaft S.sub.1.
Since the interaxial distance L is fixed in the conventional paper feed tractors shown in FIGS. 24 to 26, the deflection amount d is absorbed by clearances between the shafts and receiving members for the shafts. If the deflection amount d increases greater than the clearances, then turning torque required for rotation increases suddenly, which may cause damage to a motor or deterioration in accuracy in feeding paper and make a cause of a trouble of the paper feed tractors.
An apparatus including improved paper feed tractors of the type mentioned is disclosed in U.S. Pat. No. 4,129,239. In the apparatus, a fixing device for securing each of the paper feed tractors to a support shaft is formed as a separate unit from the paper feed tractor such that, when the interaxial distance between a drive shaft and the support shaft is varied by a deflection of the drive shaft, the fixing device unit to be secured to the support shaft may be moved relative to the paper feed tractor coupled for integral movement to the drive shaft to absorb a displacement of the drive shaft by such deformation. However, since the support shaft for the paper feed tractors is held in a stationary condition while the drive shaft is rotated in a deflected condition, the mass of elements driven to rotate in an eccentric relationship by the drive shaft which is rotated in an eccentric relationship corresponds substantially to a mass of the entire paper feed tractors. Accordingly, significant vibrations are produced by the inertia of the mass, which disadvantageously causes resonance of the entire printer to impair the printing performance of the printer and impair the performance of the paper feed tractors.